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来源:http://www.economist.com/news/science-and-technology/21679431-limiting-global-warming-2-c-above-pre-industrial-levels-more-political
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作者:Econmist
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发布时间: 2015-12-02
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1420 次浏览
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Limiting global warming to 2°C above pre-industrial levels is more a political target than a scientific one
“RESCUING our planet and our fellow man from impending catastrophe.” That was item one on Prince Charles’s to-do list for delegates attending the UN’s latest climate talks, which began in Paris this week. In his speech, he urged those present to think of their grandchildren, and to remember that they will pave “the road to a saner future”. Such sentiments have been heard many times since the inception of the United Nations Framework Convention on Climate Change (UNFCCC), an environmental treaty, in 1992. More than 190 countries have signed up to it. All agree therein to stabilise greenhouse-gas concentrations in the atmosphere so as to limit “dangerous anthropogenic interference within the climate system”.
Since 2010 they have also had an agreed target: to prevent planetary doom, global mean surface temperature must not rise more than 2°C above that of pre-industrial times. The prince reminded listeners of the fact in his speech.
That figure, 2°C, has become a touchstone. Stay beneath it, the feeling is, and all will be well—or, at least, wellish. Breach it, though, and the horsemen of the apocalypse will be unleashed in all their fury. But breached it almost certainly will be. Even if emissions do not increase from their present level (implausible in itself), enough pollution to bust through the 2°C barrier will enter the atmosphere within a mere three decades. As the horsemen’s mounts snort, then, it is worth inquiring how so much significance has come to be invested in this particular number.
Into the sausage factory
Otto von Bismarck, the first chancellor of a united Germany, is supposed to have said that laws are like sausages: it is better not to see them being made. That aphorism might apply equally to the 2°C maximum, which is a hybrid of political need and scientific haze.
It was born in the 1970s, in papers written by William Nordhaus, now an economics professor at Yale. Back then, few had heard of the idea of global warming, and fewer cared. Mr Nordhaus, who had the foresight to realise something important might be happening, suggested that a reasonable precaution would be to stop temperatures exceeding their upper bound during the past 100,000 years—the period for which ice-core data are available and for which the correlation between temperatures and other environmental effects can thus be seen reasonably clearly. The cores suggested this upper bound was 2°C above pre-industrial levels.
Since 2010 they have also had an agreed target: to prevent planetary doom, global mean surface temperature must not rise more than 2°C above that of pre-industrial times. The prince reminded listeners of the fact in his speech.
That figure, 2°C, has become a touchstone. Stay beneath it, the feeling is, and all will be well—or, at least, wellish. Breach it, though, and the horsemen of the apocalypse will be unleashed in all their fury. But breached it almost certainly will be. Even if emissions do not increase from their present level (implausible in itself), enough pollution to bust through the 2°C barrier will enter the atmosphere within a mere three decades. As the horsemen’s mounts snort, then, it is worth inquiring how so much significance has come to be invested in this particular number.
Into the sausage factory
Otto von Bismarck, the first chancellor of a united Germany, is supposed to have said that laws are like sausages: it is better not to see them being made. That aphorism might apply equally to the 2°C maximum, which is a hybrid of political need and scientific haze.
It was born in the 1970s, in papers written by William Nordhaus, now an economics professor at Yale. Back then, few had heard of the idea of global warming, and fewer cared. Mr Nordhaus, who had the foresight to realise something important might be happening, suggested that a reasonable precaution would be to stop temperatures exceeding their upper bound during the past 100,000 years—the period for which ice-core data are available and for which the correlation between temperatures and other environmental effects can thus be seen reasonably clearly. The cores suggested this upper bound was 2°C above pre-industrial levels.